In a conventional lockable pocketknife, a back spacer is rotatably attached through a spindle to the back between a pair of side plates, a blade body base end is foldably attached through a spindle to the tip portion between both side plates, and a convex portion of the end of the back spacer and an end portion dorsal concave portion engage. When closing the open blade body, the tip had to be held with the fingers of one hand while closing with the other hand in order to release the lock of the portion at which the convex portion of the end of the back spacer and the end portion dorsal concave portion of the blade body engage.
Additionally, a pocket knife is formed such that one of its pair of side plates is made into a long arm-shaped spring and is slightly bent in the direction in which a pushing pressure applies to the blade body when the knife is closed. A slight bending occurs, and the tip of the arm-shaped plate and the locking surface of the ventral portion of a tang of the blade body are fastened when the blade body of the knife is open. When closing the blade body, the blade end portion of the blade body is in the line of rotation; and when releasing the lock, the arm-shaped plate has to be pushed with one hand to release, and the blade body has to be closed with the other hand, which is extremely dangerous. Besides, there are shortcomings such as when a strong pushing pressure is applied toward the back of the blade body, a strong pushing force is applied to the arm-shaped plate of the side plate, causing bending and shortening the length of the arm-shaped plate; and that the pushing force on the locking portion of the blade body tang weakens, eliminating the locking function. There are also locking methods whereby a locking portion is attached toward the back of the knife such that fingers are not positioned in the line of rotation of the blade end portion of the blade body when closing the blade body; however, the strength of the arm shape is considered inadequate for the pushing pressure for closing the blade body.